JEE Main & Advanced Physics Simple Harmonic Motion JEE PYQ-Simple Harmonic Motion

  • question_answer
    A pendulum is executing simple harmonic motion and its maximum kinetic energy is \[{{K}_{1}}\]. If the length of the pendulum is doubled and it performs simple harmonic motion with the same amplitude as in the first case. its maximum kinetic energy is \[{{K}_{2}}\]. Then                                                       [JEE Main 11-Jan-2019 Evening]

    A) \[{{K}_{2}}={{K}_{1}}\]        

    B)                  \[{{K}_{2}}=\frac{{{K}_{1}}}{2}\]

    C) \[{{K}_{2}}=2{{K}_{1}}\]       

    D)                  \[{{K}_{2}}=\frac{{{K}_{1}}}{4}\]

    Correct Answer: B

    Solution :

    (b): \[{{K}_{1}}=\frac{1}{2}m\,{{v}^{2}}_{\max }=\frac{1}{2}m{{A}^{2}}\omega _{1}^{2}\]             ….(i)
    \[{{K}_{2}}=\frac{1}{2}m\,A_{2}^{2}\omega _{2}^{2}\]                                           ….(2)
    Here,\[{{A}_{2}}=A,\]
    From eqn. (i) and (ii)
    \[\frac{{{K}_{2}}}{{{K}_{1}}}=\frac{\omega _{2}^{2}}{\omega _{1}^{2}}\]                         \[\left( \omega =\sqrt{\frac{g}{l}} \right)\]
    \[\frac{{{K}_{2}}}{{{K}_{1}}}=\frac{{{l}_{1}}}{{{l}_{2}}}=\frac{{{l}_{1}}}{2{{l}_{1}}}\Rightarrow {{K}_{2}}=\frac{{{K}_{1}}}{2}\]


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